This invention relates to an evacuated optical structure. More particularly, this invention relates to the mounting of an optical bench in a vacuum chamber in a manner which will isolate the bench from deflections of the vacuum chamber.
An evacuated optical structure is frequently required for transporting high power laser light, because laser beam quality degrades as it passes through air. This is because small temperature variations in air can cause density variation, which tend to bend or distort the laser beam wave front. Therefore, laser beam transport structures typically are evacuated to pressures as low as 10.sup.-2 to 10.sup.-6 Torr to prevent these detrimental effects.
Laser systems also require lenses, mirrors, and other highly precise optical elements. Such optical components of the laser system are mounted to platforms, commonly called optical benches, to minimize motion. However, when the laser system, including such an optical bench, is required to be located within a vacuum vessel, the optical bench must be isolated from the distortions which a vacuum vessel experiences as it is being evacuated. Such motions are caused by the tremendous forces imposed by ambient atmospheric pressure on the boundary walls of the vessel as the air inside the vessel is being removed during pump down.
By isolating such an optical bench from such distortions of the vacuum vessel, is to permit alignment of the optical path, i.e., the optical system mirrors, lenses, etc., while the vessel is open and accessible, with no subsequent misalignment then occurring when the optics are enclosed in the vacuum system and less accessible for further adjustment.
One conventional design to achieve such isolation has been to provide an optical bench with legs which are independently mounted to a support outside of the vacuum vessel and then to provide bellows between the vacuum vessel and each leg so that any deflections of the vacuum vessel, e.g., during evacuation of the vacuum vessel, can be accommodated by the bellows without affecting alignment of the optical system mounted to the optical bench.
FIG. 1 shows such a prior art design wherein an optical bench 2 is provided with four leg assemblies (only one of which is shown), each comprising a bench plate portion 6, a pedestal portion 10, and a base portion 14 which is mounted to a support wall (not shown) external to a vacuum vessel 4 which is independently mounted to a support base. A threaded bolt 18 passes through a bore 12 in pedestal 10 and an enlarged counterbore 8 in bench plate portion 6 and is received in a tapped hole 16 in base portion 14 to thereby secure bench 2 to the support wall. Bellows 20 are mounted between vacuum vessel 4 and base portion 14 so that any movement of vacuum vessel 4, e.g., during evacuation, will be isolated from optical bench 2 by bellows 20.
While such a support and isolation design is adequate for the intended purpose of isolating movement of the vacuum vessel from the optical bench, the provision of such leg assemblies and bellows is both costly and massive. It would, therefore, be desirable if an optical bench could be mounted within a vacuum vessel, yet generally isolated from the movements of the vessel, to thereby provide a more compact and economical structure.